Iraqi forces faced further pockets of resistance from Islamic State militants in Mosul's Old City on Friday, four days after the prime minister declared victory.

Iraqi army helicopters flew overhead and explosions could be heard, residents said, while videos of alleged revenge attacks against people detained during the retaking of Mosul underlined future security challenges.

"Three mortars landed on our district," a resident of Faysaliya, in east Mosul, just across the Tigris river, said by telephone.

A few hundred Islamic State insurgents swept into Mosul three years ago, imposed a reign of terror after the Iraqi army collapsed and declared a caliphate spanning Iraqi and Syrian territory seized in a shock offensive.

The victory of U.S.-backed Iraq forces in Mosul marked the biggest defeat for Islamic State, which is under siege in the eastern Syrian city of Raqqa, its operational base.

Even though the Sunni Muslim group's caliphate is now crumbling, it is expected to revert to an insurgency and keep carrying out attacks in the Middle East and West.

Securing long-term peace in Iraq will not be easy.

Prime Minister Haider al-Abadi faces the challenge of preventing revenge killings that could create more instability, along with sectarian tensions and ethnic strife that have dogged Iraq since a U.S.-led invasion toppled Saddam Hussein in 2003.

Grand Ayatollah Ali al-Sistani, the country's top Shi'ite Muslim cleric, urged Iraqis to avoid violence and sectarianism in his first Friday sermon since the proclamation of victory in Mosul, Iraq's second largest city.

On Thursday, Human Rights Watch said it had used satellite imagery to verify that a video published on Facebook on Tuesday, showing armed men in military uniforms beating a detainee before throwing him from a height and then shooting at him, had been filmed in west Mosul.

The footage shows the men shooting at the body of another man already lying at the bottom of the perch. Reuters could not independently verify the footage.

Iraq's joint operations command said the allegations were being looked at closely and if any violations were found, those responsible would be held accountable. It also said that the videos could have been fabricated.

Three other videos posted this week by the same account appear to show members of various Iraqi security forces beating men wearing ordinary clothes. Reuters could not independently verify the footage.